1. Field of the invention
The invention relates to electrophotographic image forming devices, and in particular, structure of fusers mounted in the devices.
2. Background
Electrophotographic image forming devices such as laser printers, faxes, and copiers are equipped with fusers, which are devices for fixing a toner image on a sheet such as a piece of printing paper and a page of a document. In particular, a fuser for thermal fixing includes a fusing member and a pressing member, which are each a rotator such as a roller or a belt. A fuser equipped with rollers as both the members is referred to as a “roller-type” fuser; see, e.g. JP 2009-300472. A fuser equipped with a belt as either of the members is referred to as a “belt-type” fuser; see, e.g. JP 2009-109931, JP 2014-199309, JP 2013-160910, JP 2013-195991, or JP 2014-178555. In thermal fixing, the fusing member is heated, and at the same time, its outer peripheral surface is pressed by the outer peripheral surface of the pressing member to form a nip. Under these conditions, both the members rotate in opposite directions. Passing through this nip, a sheet is subjected to heat and pressure. Granulated toner attached on the surface of the sheet melts and permeates the surface during the passage of the sheet through the nip, and upon movement of the sheet out of the nip, the toner is cooled and solidified again. Thus, the toner image is fixed on the surface of the sheet.
Uneven toner fixing and paper wrinkling are known as typical problems for the fusers. When the fusing member is pressed by the pressing member, both the members are generally deflected in a direction normal to their axes. With these deflections, the pressure applied to a sheet during its passage through the nip between the members, which is hereinafter referred to as “nip pressure,” varies in the axial direction. When the amount of this variation is too large, toner fails to be fixed and falls off the area where the nip pressure is insufficient. This can result in deterioration in toner image quality due to rubbing off or the like. On the other hand, in the area where the nip pressure is excessive, a sheet is more likely to wrap around either of the fusing and pressing members than other areas. At the boundary between the areas, the sheet can thus deform and further wrinkle.
As ideas for preventing uneven toner fixing and paper wrinkling, the below-listed technologies are known, for example. For a roller-type fuser, a technology adjusts the force that the pressure roller applies to the fuser roller according to changes in sheet velocity caused by axial variation in nip pressure; see, e.g. JP 2009-300472. Thus, homogenized velocities of sheets can prevent uneven toner fixing and paper wrinkling In a belt-type fuser, a roller or a pressure pad or its holding member included in either of the fixing and pressing members is deflected in a direction normal to its axis when the fixing member is pressed by the pressing member. The deflection is compensated for or absorbed by difference in hardness of pressure pads, see, e.g. JP 2009-109931; difference in elasticity of holding members, see, e.g. JP 2014-199309; or undulation of the surface of a pressure pad, see, e.g. JP 2013-160910, JP 2013-195991, or JP 2014-178555. Thus, the nip pressure homogenized in the axial direction can prevent uneven toner fixing and paper wrinkling.